linux-kernel - Re: [RFC][PATCH] Fix hang in posix_locks

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Date:	Fri, 26 Oct 2007 13:07:50 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"George G. Davis" <gdavis@...sta.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] Fix hang in posix_locks_deadlock()

On Thu, Oct 18, 2007 at 02:57:59PM -0400, George G. Davis wrote:
> On Wed, Oct 17, 2007 at 02:51:57PM -0400, George G. Davis wrote:
> > ---
> > Not sure if this is the correct fix but it does resolve the hangs we're
> > observing in posix_locks_deadlock().
> 
> Please disregard the previous patch, it's not quite right - causes occasional
> segfaults and clearly did not retain the posix_same_owner() checks implemented
> in the original code.  Here's a new version which I believe retains the
> intent of the original code:
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 7f9a3ea..e012b27 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -702,14 +702,12 @@ static int posix_locks_deadlock(struct file_lock *caller_fl,
>  {
>  	struct file_lock *fl;
>  
> -next_task:
>  	if (posix_same_owner(caller_fl, block_fl))
>  		return 1;
>  	list_for_each_entry(fl, &blocked_list, fl_link) {
>  		if (posix_same_owner(fl, block_fl)) {
> -			fl = fl->fl_next;
> -			block_fl = fl;
> -			goto next_task;
> +			if (posix_same_owner(caller_fl, fl))
> +				return 1;
>  		}
>  	}
>  	return 0;

It may take multiple steps to identify a deadlock.  With the above
you'll miss deadlocks like

	process 1 is requesting a lock held by process 2
	process 2 is blocking on a lock held by process 3
	process 3 is blocking on a lock held by process 1.

Could you give more details about how you're causing
posix_locks_deadlock to hang?  Is there a simple test-case you can post?

--b.

> 
> 
> I'm not sure about those "fl = fl->fl_next; block_fl = fl;" statements,
> first, the order of those statements seems reversed to me.  Otherwise,
> I think the intent was to advance the "fl" for loop variable to the next
> entry in the list but it doesn't work out that way at all - the for
> loop restarts from the beginning - this is where we get into an
> infinite loop condition.  Whether the test case I posted before is
> valid or not, I reckon it shouldn't be possible for non-root Joe user
> to contrive a test case which can hang the system as we're observing
> with that test case.  The above patch fixes the hang.
> 
> Comments greatly appreciated...
> 
> -- 
> Regards,
> George
> -
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